66 EXPERIMENT'S on WHINSTONE and LAVA. 



cruft thus formed ferves as a pipe, within which the flowing 

 lava is confined. In feveral places on iEtna we meet with vafl 

 galleries, along which, and out of which, the lava has flowed, 

 leaving the cruft entire *. 



The irregular manner in which a lava flows, when not ex- 

 tremely heated, may likewife be referred to the fame caiife. On 

 the lower part of the running fl;ream a cruft is foi'med, fo ftrong 

 as to retard its progrefs during a certain time, but the liquid 

 behind, accumulating by degrees, at laft acquires fufficient 

 ftrength to force open the cruft ; the lava then flows out with 

 rapidity, and continues its courfe till it is again retarded by the 

 formation of a new cruft. 



These experiments feem to eftablifli, in a diredl manner, 

 what I had deduced, analogically, from the properties of whin- 

 ftone, namely, that the ftony character of a lava is fully ac- 

 counted for by flow cooling after the moft perfecft fuflon ; and, 

 confequently, that no argument againft the intenfity of volcanic 

 fire can be founded upon that charadler. We are therefore 

 juftified in believing, as numberlefs fadls indicate, that volca- 

 nic heat has often been of excefllve intenfity. 



In the coinparifon inftituted between whin and lava, the two 

 clafles are found to agree fo exadlly in all their properties which 

 we have examined, as to lead to a belief of their abfolute identity. 

 This identity has been fully eftabliflied by Dr Kennedy, who 

 has performed an exadl analyfis f of feveral of the very fjpeci- 

 mens of whinftone and lava mentioned in this paper; by which 

 he difcovered, that the elements of the two clafles are the fame: 

 above all, that they both contain 4 or 5 per cent, of foda. Their 

 agreement in this eflential circumftance feems to account for 

 their common properties, whilft the varieties of proportion, 

 among their component elements, correfpond to the flight dif- 

 ferences 



• As at Malpertui above Piedimonte. 



t An account of Dr Kenkedy's analjfis is publittied in this volume.. 



